This invention relates to a blocking apparatus for an ophthomalic lens blank of the type having a finished exteriorly disposed outer surface and an interiorly disposed inner surface capable of being machined to satisfy a given prescription, and deals more particularly with an apparatus for automatically blocking by bonding the exteriorly disposed outer surface of the lens blank to a block in precise orientation relative to reference structure on the block so that the block can be mounted directly to an automated surfacing generator where the inner surface is machined in correct orientation to the outer surface to achieve the desired prescription.
In the creation of a lens surface using automated surfacing generating systems, such as disclosed in U.S. Pat. No. 4,989,316 issued to Logan et al., data describing prescription information is transmitted to the computer of the surface generating system, and is thereafter used by the machine to cut the interiorly disposed surface of the lens to create the desired lens. The machine disclosed in this patent, as well as with other such machines that are presently in the marketplace, require that the finished outer surface of the lens blank be bonded to a block for holding the lens so that it can be placed in the surfacing machine during a cutting operation and in a lapping machine during the fining and polishing process.
Previous methods for lens blocking require manual alignment of the lens with a universal grid in accordance with axis and centering data for a prescription and marking the lens with ink to create reference marks for the actual blocking operation. At the blocking device, these marks are visually aligned with the block and a low melting point metal alloy is injected between lens and block to bond the two together. Thus, it can be seen that there are two manual alignments, the first involving visual information of a universal grid and the markings that are made on the lens relative to this grid and the second being the actual alignment of these markings with corresponding reference points on the blocking station. Among the drawbacks associated with such prior art methods is the necessity for each alignment to be made by a skilled operator. In addition, the metal alloy used to bond the lens blank to the block includes such elements as bismuth, tin, cadmium and lead, which materials are toxic and environmentally hazardous. Also, the characteristics of the molten alloy are such that the surface of the lens blank to which the alloy is bonded to, must be treated, for example, by precoating the outer surface of the lens as a means of improving adhesion of these bonding agents.
In addition, it is essential that the lens blank outer surface and the block are bonded in precise alignment with one another in accordance with prescription data because the surface generator machines the inner surface with reference to the block, and the correct prescription can be achieved only if the inner surface of the lens is aligned correctly with the outer confronting surface of the block. This relative positioning of the block and the lens opposing surfaces affects the accuracy of obtaining a desired lens thickness, since this outcome is dependent on the spacing of the block and the outer surface of the lens. Also, prismatic power depends on centering and skewing of the block on the outer surface of the lens. Cylinder power axis, required for astigmatism correction, depends on angular orientation of the block relative to any multifocal elements on the outer surface of the lens. Thus, a number of factors influence the relative positioning of the lens blank relative to the block.
Previous lens mounting blocks limited the type of lens surfaces which could be cut in the involved lens blank. That is, in these previously known blocks, the lens blank was supported by portions of the block which projected from it so that only a partial gap was provided to space the lens blank from the block. Because these projecting block portions supported the lens blank about its periphery, they did not allow the lens to be machined to a zero thickness in areas of the lens which overlie them, such as in the case of a "feathered" lens shape. Even if these projections did not interfere with such surfacing processes, the alloy bonding material which holds the lens blank to the block, would not lend itself to being readily cut by the cutting tool given its hardness and the inherent toxicity attributable to having metallic shavings released into a work environment.
It is therefore an object of the invention to provide an apparatus of the aforementioned type in which alignment of the lens blank relative to a given orientation on a blocking part is accomplished by material viewing without sighting devices thereby eliminating the heretofore known problem of viewing parallax.
Still a further object of the invention is to provide an automated blocking system whereby a user is may conduct an alignment procedure on one blank while simultaneously conducting a blocking operation on another.
A further object in the invention is to provide a system whereby prescription data describing the orientation of a lens surface to be machined relative to the block it is to be bonded to is stored in a host computer and is on-demand downloaded from the host to an apparatus of the type heretofore discussed.
It is still a further object of the invention to provide a machinable bonding agent for bonding in a lens blank and block assembly so as to support the lens blank such that up to zero thickness cuts can be made in the blank about its periphery without cutting the block.
Yet still a further object of the invention is to provide an apparatus capable of the bonding a lens blank with the block using various bonding agents, including low melting point thermoplastic, through management of temperature and pressure during the injection and curing cycle and to provide such a bonding agent which eliminates the need for pre-coating the outer surface of the lens as a means of improving adhesion of the bonding agent.
Another object of the invention is to provide a blocking system which provides a uniform support for the lens blank to assure aberration free surface generation and polishing.
Still a further object of the invention is to provide a block position sensing support which during a bonding operation detects incorrect positioning of the block in the apparatus thereby stopping the process to avoid blocking in unwanted prismatic power and incorrect lens thickness.
A further object of the invention is to provide a block positioning support whereby the block is automatically moved to a designated angular orientation to align the prescription cylinder axis.